In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. Often, such control systems have a centralized control room, with computer systems having user inputs and outputs and having peripheral systems that are known in the art, such as printers, scanners, and the like. Generally, a controller and process subsystems are coupled to the computer systems.
Typically, control systems are distributed such that field devices are separated or geographically removed from the control room. The process subsystem is connected to the field devices. As used herein, the term “field device” encompasses any device that performs a function in a distributed control system and is known in the control art.
Generally, each field device includes a transducer. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Often, one system provides power to actuate a transducer, which in turn supplies power usually in another form to a second system. For example, a loudspeaker is a transducer that transforms electrical signals into sound energy. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and the like.
Traditionally, analog field devices have been connected to the process subsystem and the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the controller.
Traditional discrete devices transmit or respond to a binary signal. Typically, discrete devices operate with a 24-volt signal (AC or DC), a 110 or 240 volt AC signal, or a 5 volt DC signal. Of course, a discrete device may be designed to operate according to any electrical specification required by the control environment.
While historically field devices were capable of performing only one function, recently hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) and the Instrument Society of America (ISA) Fieldbus SP50 standards superimpose a digital carrier signal on the current loop signal. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions.
Additionally, many field devices contain circuitry for grooming the sensed process variable. Often, the field device includes a sensor, an Analog-to-Digital (A/D) converter and a processor, which is used to groom the signal into a 4 to 20 mA or 1 to 5 volt output. The term “grooming” refers to linearization, temperature compensation, trimming, scaling, or otherwise evaluating the raw A/D signal. The “grooming” process can be modeled as an algebraic equation, according to the specific type and environment of each specific field device. Accordingly, the “grooming” process may vary from one field device to the next, so as to account for specific environmental conditions.
For highly distributed monitoring applications, wireless transmitters are possible. However, implementation of wireless systems has been limited due to limited coverage area, high power consumption, and cost considerations. Specifically, adding a wireless transmitter to the sensor/grooming circuit of the prior art introduces an additional wireless communications card, which converts an analog signal into a digital signal in order to send it over the wireless communication system (e.g. PCS, and the like). Generally, the data from the sensor is already digital and must be converted to an analog signal prior to sending it to the wireless communications board, which then converts the analog signal back into a digital signal. The introduction of the wireless communications board not only increases the amount of circuitry in the field devices, and therefore the cost, but the additional circuitry increases the power consumption of the field device.
While the wireless solution is desirable, power consumption is still too high, and the additional cost of adding a wireless transceiver to each monitoring point is still too high. There is a need for low-cost, low-power consumption, wireless, distributable field devices.